Recently, a WDM optical communication system, in which plural signals with different wavelengths are multiplexed and transmitted through a single optical transmission line, is being vigorously developed as means for realizing an optical transmission system with high capacity.
In order to extend the transmission distance of the WDM optical communication system, it is important that the transmission levels of the respective optical signals are as high as possible and equal to each other. Accordingly, following methods have been adopted to achieve the aforementioned purpose until now.
The first method is that outputs of plural signal light sources of different wavelengths are multiplexed by a coupler, and the signal light sources are so adjusted that the power levels of the optical signals are equalized at the output port of the coupler.
The second method is that optical variable attenuators are respectively inserted between the signal light sources and the coupler, and the optical variable attenuators are so adjusted that the power levels of the optical signals are equalized at the output port of the coupler.
The third method is that optical fiber amplifiers are respectively inserted between the signal light sources and the coupler, and the gains of the optical fiber amplifiers are so adjusted that the power levels of the optical signals are equalized at the output port of the coupler.
However, in the first and second methods, since the power levels of the optical signals are adjusted based on the minimum power level of them, so that the high transmitting power levels cannot be obtained.
According to the third method, since each of the signal light sources are respectively provided with the optical fiber amplifier, the system is magnified and high-priced.